For nylon messenger bags they are ideal for small and lighter things like some documents, an iPad and some other things in between. They very easy to carry as you can swing them from the side when you need something or when taking public transport its easier to protect it from theft. Provided the nylon messenger bags has a zipper and not just a flip cover to start with. Plus in my case carry portfolios and a tablet is the most common gear i bring along so this bag fits the bill. although I prefer the nylon messenger bags that are monochrome, plain, leather than some of the flashy designs.
For backpacks, it would be ideal for the larger items such as laptops, camera, and some accessories. these would be ideal with backpacks as they are bigger, sturdier, and the ergonomics for carrying them is much more ideal..For sure I have seen camera bags that are messenger style but letting just one shoulder carry the weight is not ideal and not healthy. a swinging bag is not good for electronic gear. my bags are always plain and relatively has anti theft features. most of the time people around me in public will never thought of me carry expensive gear…As I always try to avoid getting the unwanted attention.
Again, its all about personal style and were you are comfortable. I always espouse that you decide on how you choose your gear. For me as long as its logical then I have no problems which to use.
Moreover, no matter messenger bag or backpack, you could always find a suitable gear to match your style from DDHBA, one of the top 10 backpack factory for business bag.